Various dispensing systems have been used in the past for applying patterns of viscous liquid material, such as hot melt adhesives, onto a moving substrate for a wide range of manufacturing purposes, including but not limit to packaging, assembly of various products, and construction of disposable absorbent hygiene products. Thus, the dispensing systems as described are used in the production of disposable absorbent hygiene products such as diapers. In the production of disposable absorbent hygiene products, hot melt adhesive dispensing systems have been developed for applying a laminating or bonding layer of hot melt thermoplastic adhesive between a nonwoven fibrous layer and a thin polyethylene backsheet. Typically, the hot melt adhesive dispensing system is mounted above a moving polyethylene backsheet layer and applies a uniform pattern of hot melt adhesive material across the upper surface width of the backsheet substrate. Downstream of the dispensing system, a nonwoven layer is laminated to the polyethylene layer through a pressure nip and then further processed into a final usable product.
In various known hot melt adhesive dispensing systems, continuous filaments of adhesive are emitted from a plurality of adhesive outlets with plural process air jets oriented in various configurations adjacent the circumference of each adhesive outlet. The plural air jets discharge air in a converging, diverging, or parallel manner relative to the discharged adhesive filament or fiber as the filament emerges from the adhesive outlet. This process air can generally attenuate each adhesive filament and cause the filaments to move in overlapping or non-overlapping patterns before being deposited on the moving substrate.
Manufacturers in many fields, including manufacturers of disposable absorbent hygiene products, are interested in small fiber technology for the bonding layer of hot melt adhesive in nonwoven and polyethylene sheet laminates. To this end, hot melt adhesive dispensing systems have incorporated slot nozzle dies with a pair of air channels formed on each side of the elongated extrusion slot of the die. The air channels are angled relative to the extrusion slot and arranged symmetrically so that curtains of pressurized process air are emitted on opposite sides of the extrusion slot. Thus, as hot melt adhesive is discharged from the extrusion slot as a continuous sheet or curtain, the curtains of process air impinge upon and attenuate the adhesive curtain to form a uniform web of adhesive on the substrate.
Meltblown technology has also been adapted for use in this area to produce a hot melt adhesive bonding layer having fibers of relatively small diameter. Meltblown dies typically include a series of closely spaced adhesive nozzles or orifices that are aligned on a common axis across the die head. A pair of angled air channels or individual air passages and orifices are positioned on both sides of the adhesive nozzles or orifices and aligned parallel to the common nozzle axis. As hot melt adhesive discharges from the series of aligned nozzles or orifices, pressurized process air is discharged from the air channels or orifices to attenuate the adhesive fibers or filaments before they are applied to the moving substrate. The air may also cause the fibers to oscillate in a plane that is generally aligned with the movement of the substrate (i.e., in the machine direction) or in a plane that is generally aligned in the cross-machine direction.
One of the challenges associated with the above-described technologies relates to the production of fibrous adhesive layers during intermittent operations. More specifically, for some applications it is desirable to produce discrete patterns of fibrous adhesive layers rather than a continuous adhesive layer. Although known fibrous adhesive dispensers incorporate intermittent control of the adhesive and air flows to produce such discrete patterns, providing the discrete patterns with well-defined edges can be difficult to achieve.
For example, the velocity of the air directed at the adhesive must be sufficient to cleanly “break” the filaments when adhesive flow is stopped. Otherwise the filaments may continue to “string” along so that there is no clearly defined cut-off edge and cut-on edge between adjacent patterns deposited on the moving substrate. When high velocity air is used, however, the pattern of fibers between the cut-on and cut-off edges becomes more difficult to control. This is particularly true when high velocity air flows converge to impinge opposite sides the adhesive filaments. The filaments may end up breaking constantly during the dispensing cycle rather than merely at the starting and stopping points of the adhesive flow.
A related problem resulting from high velocity air directed in this manner is “fly,” which occurs when the adhesive gets blown away from the desired deposition pattern. The “fly” can be deposited either outside the desired edges of the pattern, or even build up on the dispensing equipment and cause operational problems that require significant maintenance. High velocity air, in combination with closely spaced nozzles, can also cause “shot” in which adjacent adhesive filaments become entangled and form globules of adhesive on the substrate. “Shot” is undesirable because it can cause heat distortion of delicate polyethylene backsheet substrates.
As can be appreciated, known adhesive dispensers that produce continuous, fibrous adhesive layers may not be particularly suitable for intermittent operations. Therefore, there remains room for improvement in this area of fibrous adhesive dispensing technology.